The present invention relates to a method and apparatus for establishing, maintaining and accelerating hydrocarbon production from offshore reservoirs. More particularly, the present invention relates to a method and system for economically developing deepwater oil and gas prospects with surface accessible completions.
Traditional bottom-founded platforms having a fixed or rigid tower structure have been taken to their logical depth limits in the development of offshore oil and gas reserves. Economic considerations suggest that alternatives to this traditional technology be ordinarily used in waters deeper than about 1200 feet in the Gulf of Mexico and often less in other areas. Further, even the most promising reservoirs are difficult to economically exploit in this manner at any greater depth.
One alternative to fixed towers is to drill from facilities provided on surface vessels and to complete the wells at the ocean floor with subsea completions. Gathering lines connect the subsea wells to facilities usually located at the surface, either in the immediate vicinity or provided remotely in a satellite operation.
However, subsea wells are relatively inaccessible at the ocean floor and this fundamental problem is exacerbated by the rigors of the maintenance-intensive subsea environment. The result is complex, costly maintenance operations which are difficult to accomplish with either through-flow line tools or the remotely operated vehicles or manned submarines suitable for deepwater applications. Further, maintenance is impossible for divers in all but the most shallow of deepwater applications and even there it is both dangerous and difficult work.
Alternatively, deepwater wells can be provided with surface completions on specialized structures more suitable for deepwater applications. Designs have been developed for various configurations of tension leg, compliant tower, and articulated tower platforms as well as floating production systems which can provide drilling and production facilities in deepwater at costs not possible for traditional fixed platforms. Nevertheless, the high cost of these structures requires a high concentration of wells in traditional practice in order to be economically feasible. Many hydrocarbon reservoirs cannot effectively utilize, and therefore justify, such a number of wells. Other reservoirs can justify the number of wells, but only if extended reach drilling techniques are used to drain relatively remote areas of the reservoir from the facilities provided on the platform. This extended reach can be accomplished with the current directional and horizontal drilling techniques, but only by substantially increasing the drilling cost for the wells so extended.
The cost of deepwater platforms further increases if the drilling operations are to be conducted from the platform itself. This substantially increases the load on the platform, thereby requiring a substantially larger structure. Further, primary drilling operations to develop a dispersed reservoir with extended reach techniques from a central location can spread the drilling operations over many years. Subsequent well workover operations may tie the drilling rig to the platform many years thereafter even though primary drilling is complete. Both aspects represent economic inefficiencies. In the first instance, drilling such extended reach wells, one well at a time, delays and defers production, thereby adversely affecting the rate of return of the substantial capital expenditures necessary to provide such a deepwater structure. Further, after the wells have been drilled, the rig represents a very substantial asset which cannot otherwise be efficiently used and has similarly permanently committed the prospect to the larger structure, thereby affecting the cost of the platform as well.
Alternatively, the wells can be predrilled from a drill ship or other floating facility, killed or otherwise secured, and completed from a scaled-down "completion" rig carried on a production platform such as a tension leg well platform (TLWP) installed at the site later. This reduces the load on the permanent facilities and therefore permits a smaller platform, but prevents production from any well until all the wells have been drilled and thereby substantially defers revenue from the development. Further, this scheme does not allow the flexibility to permit additional or replacement drilling once the platform has been installed.
Efficient development of deepwater hydrocarbon reserves must overcome these deficiencies and provide a method and system for developing the reservoirs with lower capital outlays, faster return on investment, more efficient reservoir management for larger reservoirs, and enhanced profitability for reservoirs that are otherwise marginal.